The present invention relates to a method and a device for optical amplification and, more particularly, to a method and a device for directly amplifying wavelength division multiplex (WDM) optical signals for use in an optical communication system.
Optical communication systems include a WDM communication system in which a plurality of WDM optical signals each having a particular wavelength are sent together through an optical fiber or transmission path. In this kind of system, the multiplexed optical signals are amplified by the following procedure.
The multiplexed optical signals are incident to an optical direct amplifier or amplifying medium implemented by, e.g., an erbium-doped optical fiber. The direct amplifier amplifies the incident optical signals in the form of light. The amplified optical signals are input to a beam splitter and split into two thereby. One part of the split optical signals is applied to an optoelectrical converter and transformed to an electric signal thereby. The electric signal is input to a gain controller. In response, the gain controller controls the gain of the direct amplifier such that the electric signal has a preselected level.
As stated above, it has been customary with a method and a device for optical amplification to control the gain of the optical direct amplifier such that the multiplexed optical signals have a preselected total output level. Hence, so long as the conventional device deals with a fixed number of multiplexed optical signals, it can perform accurate control over the optical output level.
However, it is difficult with the conventional device to change the preselected number of optical signals afterward. For example, when the number of optical signals to be amplified is increased, the output level of the individual signal decreases because the device controls the total optical output level to a preselected level. As a result, the individual optical signal cannot be amplified to the expected level.
In this manner, it is difficult with the conventional method and device to change the number of signals to be multiplexed, i.e., the transmission capacity after the start of operation of the system. In this sense, the system using the conventional method and device for optical amplification lacks in extendibility.